An immune response. Colostrum contains transfer factors which can cause the immune system to go on alert. Reactions can include feeling flu like, fatigue, sore throat, headaches, and swollen lymph nodes. If the immune system goes on a killing spree of pathogens, the body can purge the dead toxic pathogens and a Herxheimer Effect will ensue. It can also include the above symptoms and profuse sweating, nausea, and diarrhea.

As bad as this sounds, once these symptoms pass, one usually feels better. It is a good idea to talk to one's doc before taking any supplement. There are some conditions, such as cancer and autoimmune illnesses, where it may not be desirable to rile up the immune system.